It is known that windings of power transformers—for example with a rated power of 300 MVA, a rated voltage of 380 kV on the primary side and a rated voltage of 1 10 kV on the secondary side—typically are fixed by a so called winding clamping in order to ensure a sufficient insulation distance in between adjacent winding loops. In case of insufficient winding clamping the active parts of the transformer are not disposed for the mechanical stresses caused by electrical stresses due to high currents occurring in electrical energy distribution networks. In case of short circuits or even a high variation of the load current strong mechanical forces in between the adjacent winding loops are subject to occur. To prevent any mechanical movement respectively deformation of the winding respectively its winding loops the winding is clamped with a respective pressure force at its axial ends. The pressure force in radial direction is mostly withstood by the winding loops itself. Distance elements, for example made from pressboard, are foreseen in between radial adjacent winding layers and/or in between axially adjacent winding loops in order to ensure a sufficient insulation distance and in order to fix the winding structure against mechanical movement.
Due to the occurring forces in between the loops of the winding the winding clamping of a transformer is subject to age over the years of operation, so that one day the functionality might not be sufficient enough to ensure a fault free operation of a respective transformer. In this case the pressure force applied on the winding is typically too low, so that the winding loops are not fixed in a sufficient manner any more.
The aging of a winding clamping of a transformer is not subject to a continuous aging process which is predictable in a good way. Moreover aging is subject to the individual mechanical stress impact on each winding of a respective transformer and is not calculable in a good way therewith. Thus in several cases an individual analysis of the status of the winding clamping of a transformer is required in order to prove whether it is still sufficient or not. In case of a not sufficient status of winding clamping a retrofit has to be done in order to ensure a fault free operation of the transformer for the future.
Analysis of a winding clamping is a rather difficult task since power transformers typically are arranged within an oil filled transformer tank and are not easily accessible therewith. Thus a direct assessment of the winding clamping, for example based on a visual or manual control, is not feasible without removing the power transformer from its oil filled transformer tank.
Assessment methods are known where the winding respectively the winding clamping is mechanically respectively electrically exited to vibration, either by impact of a mechanical force impulse or by applying a current pulse on the winding to be assessed. The winding to be assessed is arranged on the transformer core of the power transformer. Due to the residual magnetism of the transformer core a voltage is induced in the winding when it is vibrating. Vibration of the winding strongly depends on the status of the winding clamping.
The patent document RU 21 17955 discloses a method for analysis of a respective induced voltage, wherein the coil of the power transformer is excited to vibration by a mechanical force impulse on its oil tank. The induced voltage is measured and transferred into the frequency domain. It is assumed that the induced voltage features a peak in the frequency domain at a predetermined frequency which corresponds to the resonance frequency of the winding. Based on the amplitude of this peak a formula is provided for calculating the remaining pressure force of the winding clamping as indicator for the status of the winding clamping.
Disadvantageously within the state of the art is that power transformers typically do not have only one single resonance frequency: power transformers typically have several windings, at least a primary and a secondary winding, which are subject to different pressure forces and to different resonance frequencies therewith. Thus the method according to prior art is subject to fail in many cases.